Project Summary Innate social behaviors such as aggression, mating, and parenting offer a powerful window on the brain networks that map sensory input onto appropriate motor outputs. The compact architecture of the rodent vomeronasal system is ideal for characterizing the circuits that translate identified, sex-specific chemosignals into responses matched to the characteristics of different conspecifics. Here we address two seemingly contradictory roles of this system: the need to mediate stereotyped interactions with same- or opposite-sex partners, while still providing flexibility in responses to different individuals based on established social relationships. We characterize the learning mechanisms that mediate changes in aggression, a prototypical and highly robust male behavior that is powerfully altered by dominance relationships. Using powerful genetic tools to access functionally distinct sensory pathways in the accessory olfactory bulb, our experiments will address (1) the cellular plasticity processes that adapt aggression levels to reflect social experience, (2) the way that familiarity shapes the sensory representations of conspecifics during active social interactions, and (3) the circuit anatomy that organizes behaviorally relevant sensory signals and routes them to downstream limbic centers. A deeper understanding of the neural mechanisms for regulating aggression may help design strategies for mitigating pathological behaviors in human communities. More broadly, a deeper knowledge of the brain networks for social behaviors is highly relevant for understanding autism spectrum disorders.